1. Field of the Invention
The present invention relates to an analog amplifier and analog filter for amplifying an analog signal. More particularly, the present invention relates to an amplifier and filter having a cutoff frequency controlled according to a digital control code.
2. Description of the Related Art
FIG. 1 illustrates a configuration of an analog filter according to the related art.
Referring to FIG. 1, an analog filter is configured by combining a plurality of filter stages 100 of a first or higher order. A high-pass feedback stage 110 is connected between a first amplifier stage and an (n−1)th amplifier stage, and removes a noise included in a Direct Current (DC) component and a DC offset.
Each of the filter stages 100 includes an operational amplifier, a variable resistor, and a variable capacitor, and the gain and cutoff frequency thereof are controlled by the variable resistor and the variable capacitor. That is, the gain of each filter stage 100 is determined by a ratio of an input resistance to a feedback resistance, and the cutoff frequency is inversely proportional to a product of a feedback resistance and a feedback capacitance.
The variable resistor of each filter stage 100 may include two or more segments each configured by combining short switches and a plurality of resistors, and the short switches are controlled by a digital code. The digitally controlled serial resistor connection has a binary structure in which the number of resistors increases, such as 2R, 4R, 8R, 16R, . . . , 2nR (where n is an integer), and the total resistance is linearly proportional to the digital code. The resistance of the variable resistors varies linearly according to the digital code K, and the cutoff frequency is proportional to a reciprocal of the resistance.
Alternatively, the variable capacitor of each filter stage 100 may include two or more capacitor segments each configured by combining short switches and capacitors in order to variably change the cutoff frequency, and the short switches may be controlled by a digital code. For example, the digitally controlled parallel capacitor connection has a binary structure in which the number of the capacitors increases, such as 2C, 4C, 8C, 16C, . . . , 2nC, and the total capacitance of the capacitors is linearly proportional to the digital code. The total capacitance of the variable capacitors varies linearly according to the digital code K, and the cutoff frequency is proportional to the reciprocal of the capacitance.
Generally, a frequency axis is represented on a logarithmic scale in the frequency domain, and a decibel (dB) unit representing a gain has a logarithmic scale value. Accordingly, a variable resistance or capacitance, which varies linearly according to the digital code K, has nonlinear characteristics, and thus reduces efficiency.
That is, as the value of the digital code K increases, the variable resistance or capacitance varies rapidly on the logarithmic scale, whereas as the value of the digital code K increases, the variable resistance or capacitance varies slowly on the logarithmic scale. This reduces the accuracy of the variable resistance or capacitance upon operation at high frequency bands as illustrated in FIG. 2, causing uncontrollable intervals as well as the reduction of efficiency.
FIG. 2 is a graph illustrating a relation between a frequency and a gain according the related art.
As the value of the digital code K increases, the variable resistance or capacitance varies slowly on the logarithmic scale, whereas as the value of the digital code K increases, the variable resistance or capacitance varies rapidly on the logarithmic scale, causing intervals in which the cutoff frequency is uncontrollable.
Also, when the frequency change widths of respective digital codes are measured, the change widths are inconsistent due to quantization errors of respective change intervals as illustrated in FIG. 3, so that uncontrollable intervals occur even though the frequency axis is considered linearly rather than on the logarithmic scale.
FIG. 3 is a graph illustrating a relation between a digital control code and a frequency according the related art.
Accordingly, there is a need for a filter and amplifier having a cutoff frequency conveniently controlled exponentially according to a digital code.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.